In the operation of internal combustion engines with fuel injection, especially diesel engines, both the timing and the quantity of fuel injection are important in obtaining the desired performance. In some engines it is necessary to inject large quantities of fuel at high pressure. Further, the operating parameters which determine the optimum timing for injection may vary independently of those parameters which determine the optimum quantity of fuel with such variations occurring from one engine cycle to the next. It is presently known to utilize an electronic computer to develop control signals for injection timing and metering which can be changed from one engine cycle to the next in accordance with engine operating parameters, such as speed setting, manifold vacuum, atmospheric pressure and the like.
In the prior art, it has been proposed to provide a fuel injector in which the timing of the injection is controlled independently of the fuel quantity of the injection. Such an arrangement is shown in the Bassot et al U.S. Pat. No. 3,516,395. In the injector of this patent an electromagnetic valve is opened for a controlled time period to admit fuel under constant pressure to a metering chamber for injection. The timing of the initiation of injection is controlled by an engine operated 3-way valve which admits fluid pressure to the servo cylinder to actuate the injection piston. In the Hussey et al Pat. No. 3,587,547 the timing and metering of the injection are controlled separately, the metering being performed by opening a fuel distributing valve for a controlled time period. It has also been proposed to control the metering of fuel for each injection by controlling the length of the injection stroke of the injector piston by controlling the quantity of the actuating fluid for the piston. This is described in the Sampietro U.S. Pat. No. 2,946,513.
Fuel injectors utilizing an electromagnet for controlling the timing and metering of the fuel injection are well known in the prior art. The Komaroff U.S. Pat. No. 3,623,460 discloses an injector in which the injector piston is actuated directly by an electromagnet on both the forward and reverse strokes. On the reverse stroke, the fuel is metered by the amount of time that the piston is retracted from the neutral position and injection is initiated by the forward stroke. In the injector of the Links U.S. Pat. No. 3,835,829 a single solenoid valve controls the metering and the timing of the fuel injection. When the solenoid is de-energized fuel flows into the pump chamber and when it is energized the fuel pressure energizes the servo piston and causes injection to occur. In the injectors described in these patents, the metering depends upon the length of the time period and the value of the supply pressure during which fuel flows into the metering chamber. Solenoid controlled injectors are also show in the Monpetit et al U.S. Pat. No. 3,680,782 and the Reneault et al U.S. Pat. No. 3,802,626.
It is known to provide an injector with a spool valve for controlling the energization of the servo piston and to employ a pilot valve to control the spool valve, as shown in Takahashi et al U.S. Pat. No. 3,943,901. In the injector of this patent a single pilot valve, either a monostable or a bistable fluid element, energizes the servo piston through the pilot valve when it is in one position and it actuates the spool valve to deenergize the servo piston when the pilot valve is in the other position. The timing and the quantity of fuel injection both depend upon the duration of the control signal on the pilot valve.
An objective of the present invention is to provide an injector which overcomes certain disadvantages of the prior art, especially in respect to the manner and precision with which timing and metering is executed for each injection of fuel.